The late Paleozoic El Imperial Formation,western Argentina: Glacial to post-glacial transition and stratigraphic correlations with arc-related basins in southwestern Gondwana

The El Imperial Formation of the San Rafael Basin records a succession of depositional environments during the latest Mississippian to earliest Permian that span before, during, and after the glaciation of west central Argentina. At the base of the formation, a restricted marine environment is recorded in mudstone containing marl and rippled and deformed sandstone beds. This unit, or sequence 1, is incised by a deltaic facies association composed of cross-bedded sandstone and conglomerate that form at least 5 stacked Gilbert deltas. The deltaic facies association grades upward into the glacially-influenced facies association, made up of stratified diamictite, mudstone with dropstones, and massive deformed sandstone, indicating deposition by wet-based tidewater glaciers that calved icebergs into the basin, with contributions from mass movement processes. The glacially-influenced facies association is overlain by mudstone and horizontally laminated and cross-bedded sandstone of the post-glacial open marine facies association, recording post-glacial transgression followed by relative sea level fall. The deltaic, glacially-influenced, and post-glacial open marine facies associations comprise sequence 2. Sequence 2 is incised by conglomerate of the upper fluvial member, or sequence 3.The strata of the El Imperial Formation are correlated to those of the other arc-related basins of western Argentina: Río Blanco, Calingasta–Uspallata, and Tepuel. A Bashkirian transgression and fluvial incision in the El Imperial Formation correlate with events in the Río Blanco and Calingasta–Uspallata Basins to the north, whereas glaciation continues to the south in the Tepuel Basin through the Early Permian. The deviating stratigraphic record of the Tepuel Basin may be the result of its higher latitudinal position during the Pennsylvanian–Early Permian and higher altitude due to either tectonic convergence of the Patagonian microplate or convergence along the Panthalassan margin of southwestern Gondwana.     

西南三江临沧花岗岩基成因与构造启示:元素地球化学、锆石U-Pb年代学及Hf同位素约束

昌宁-孟连缝合带系中国西南三江特提斯造山带一条重要的古特提斯主洋残余,在洋盆消减过程中发生了强烈岩浆作用,形成了以临沧花岗岩基为代表的平行缝合带的大规模物质-热活动产物。其为理解古特提斯洋俯冲和闭合过程提供了良好窗口。本文对出露于临沧岩基中部临沧地区的花岗岩类进行了全岩元素地球化学、锆石U-Pb年代学和Hf同位素组成研究。实验样品锆石~(206)Pb/~(238)U加权年龄为215.5±0.4Ma(MSWD=2.8,n=23),反映岩浆岩侵位于晚三叠世。花岗岩主要属于高钾钙碱性系列,岩性为花岗闪长岩和二长花岗岩,高铝饱和指数(A/CNK=1.09～1.17)和刚玉分子数(1%)显示了过铝质特征。样品富集轻稀土元素(LREE/HREE=6.29～17.01,(La/Yb)_N=6.36～33.5)和大离子亲石元素,亏损Nb、Ta等高场强元素。锆石Hf同位素分布集中,ε_(Hf)(t)均为负值(-15.9～-9.08),Hf地壳模式年龄较老(1.6～1.9Ga),全岩Zr饱和温度(734～779℃)较低,指示了临沧花岗岩主体系古老地壳基底部分熔融产物,无明显地幔物质加入。S型花岗岩源区包括泥质岩和硬砂质岩两种,本文研究临沧花岗岩源区主要为硬砂质岩。明显Eu负异常(δEu=0.19～0.58)和较低分异指数(DI=73～82),反映源区存在斜长石和角闪石等的残留。文章进一步总结并分析了前人岩石地球化学和同位素数据,结果表明临沧岩基花岗岩类岩石成因类型复杂多变。由晚二叠世到晚三叠世(260～200Ma),古特提斯洋由俯冲到闭合过程中,依次演化发育S型、I型和A型多种类型花岗岩。综合临沧岩基两侧与之平行的高压变质岩类和双峰式火山岩地质特征,认为临沧花岗岩类主体形成于古特提斯洋同碰撞造山(250～237Ma)和后碰撞伸展(235～203Ma)两种构造环境,早期存在少量与洋盆俯冲作用相关的岩浆岩(～252Ma)。     

东天山晚古生代构造转折: 红山南-天木东地区岩浆岩年代学与地球化学的约束

东天山是中亚造山带的重要组成部分, 区内晚古生代岩浆活动与成矿作用强烈, 是理解中亚造山带构造演化与成矿作用的关键地区。然而, 前人对东天山构造带由俯冲向碰撞转变的时间和过程仍存在较大争议。本文对东天山红山南-天木东地区广泛出露的晚古生代岩浆岩开展了野外考察和岩相学鉴定, 进行了年代学和岩石地球化学分析, 以限定其形成时代、岩石成因和构造背景, 进而探讨晚古生代构造演化过程。红山南-天木东地区岩浆岩主要为早石炭世火山岩和侵入岩, 次为早二叠世侵入岩。早石炭世火山岩主要为安山岩(328.8±2.0Ma)和英安岩, 侵入岩主要为辉长闪长岩(328.7±1.8Ma); 早二叠世侵入岩为黑云母二长花岗岩(290.3±2.1Ma)和石英闪长岩(290.0±1.6Ma)。早石炭世岩浆岩富含角闪石和斜长石, 为钙碱性, 准铝质系列, 富Rb、Ba、Th、U和Pb等大离子亲石元素, 亏损Nb、Ta和Ti等高场强元素和Sr-Nd-Hf同位素, 具有岛弧岩浆岩特征, 是交代地幔楔部分熔融的产物。相对于早石炭世岩浆岩, 早二叠世侵入岩富含黑云母和碱性长石, 富集SiO₂、Na₂O和K₂O, 贫Al₂O₃、MgO、Fe₂O₃^T及CaO, 同位素更亏损, 为碰撞后背景下新生加厚地壳部分熔融的产物。总之, 红山南-天木东地区的早石炭世与早二叠世岩浆岩地球化学差异显著, 指示东天山构造背景从早石炭世大洋俯冲体制转变为早二叠世碰撞后造山体制, 即其构造转折时间为晚石炭世-早二叠世。

     

Pulsating subsidence during a rift episode: stratigraphic and tectonic consequences (Aptian–Albian, northern Spain)

Backstripping analysis has been carried out on nine outcrop sections of the Basque-Cantabrian Aptian and Albian of northern Spain which consist of shallow-water marine deposits up to 7000 m thick. It reveals four main pulses of common accelerated subsidence: I (Early Aptian), II (Early Albian), III (lower Late Albian), and IV (uppermost Late Albian) separated by tectonic quiescent intervals. Tectonic subsidence rates of 70 m Myr^–1 for the Aptian and 101 m Myr^–1 for the Albian were recorded (Sopuerta section). The calculated stretching factor referred to a pre-rift continental crust of 35 km is (β_s=1.32) and represents approximately 8.5 km of crustal thinning. Points of change towards lesser subsidence rate in the curves are correlated with major unconformities in the sections, in the Basque-Cantabrian Basin and in Boreal Europe, suggesting geodynamic changes related with the opening of Bay of Biscay. Minor variations of spreading direction and intensity are proposed to explain the subsidence pulses.     

Autogenic dynamics of alluvial fans in endorheic basins: Outcrop examples and stratigraphic significance

Alluvial fans are relatively simple depositional systems, due to the direct coupling of sediment sources and adjacent accumulation areas. Nonetheless, general models of alluvial‐fan evolution and stratigraphy remain elusive, due to the great sensitivity of such systems to allogenic controls and their strongly case‐specific responses. Autogenic processes intrinsic to alluvial‐fan dynamics can complicate stratigraphic architectures, with effects not easily distinguishable from those of allogenic forcing. A distinction is made here between lateral autogenic dynamics, tied to spatial sediment distribution over fan surfaces, and vertical autogenic dynamics, related to independent incision‐aggradation cycles. Autogenic mechanisms have been highlighted recently by modelling studies, but remain poorly constrained in field‐based studies. Examples are presented here from the margins of the Cenozoic Teruel and Ebro basins (Spain), where alluvial fans accumulated thick successions during phases of basin topographic closure and endorheic drainage which promoted forced aggradation. Fan successions consist of conformable architectures of stacked clastic sheets, laterally continuous and with no evidence of internal unconformities, inset architectures, fan segmentation or preserved incised channels. Continuous aggradation in these closed basins strongly inhibited ‘vertical’ autogenic dynamics in the form of fan head and through fan incision, due to the forced rise in geomorphic base level and the creation of positive accommodation. Furthermore, the lack of incised channels favoured widespread sediment transport and aggradation over broad fan sectors in relatively short time spans, in contrast to the typical occurrence of active lobes and abandoned fan surfaces caused by ‘lateral’ autogenic dynamics. Stratigraphic records of alluvial fans developed in endorheic basins are essentially complete and largely unaffected by autogenic processes. The latter characteristic implies that they can be more unambiguously interpreted in terms of allogenic forcing, because stratigraphic signatures are not complicated by the effects of complex fan autodynamics.     

剥蚀与沉积的相关性是陆相地层划分的基础

陆相盆地由于纵横向岩性突变和缺乏化石,导致地层划分往往十分混乱。陆相红盆的沉积始终离不开基底母岩的供给。笔者通过剥蚀与沉积的相关性研究实例,来说明其在陆相盆地划分和研究中的实际作用和意义。把盆外母岩的风化剥蚀与盆内堆积的相关性关系研究紧密结合起来,可以极大地提高陆相地层划分的分辨率;同时也有助于加强盆地构造活动的研究。     

南海北缘新生代构造演化的深部制约(I):幔源包体

对南海北缘新生代玄武岩中幔源包体的研究揭示了该地区上地幔的不均一性和复杂的地幔过程。部分橄榄岩中单斜辉石(Cpx)具有极低的Ti含量(<160μg/g),这与这些样品中较高的Cpx含量(8%～12%)不匹配。主元素成分变化趋势以及Cpx中HREE分异现象,暗示该区上地幔经历了变压熔融。地幔熔融始于深部的石榴子石稳定区并延续到浅部的尖晶石稳定区(总熔融程度达23%)。这些熔融残余受到了硅酸盐和含水流体的交代。橄榄岩的微量元素组成指示南海北缘地幔具有大陆裂谷型地幔的特征,而与俯冲带之上的地幔楔或受俯冲作用影响的地幔有很大的差别。包体的平衡温度和橄榄石中Fo之间的负相关关系暗示岩石圈地幔具成分分层结构。该地幔的下部由大洋型橄榄岩组成,而地幔顶部为类似于太古宙-元古宙地幔的富斜方辉石方辉橄榄岩。这一岩石圈结构与该地区岩石圈的减薄和软流圈对老岩石圈的置换有关。深源岩石记录的信息支持南海海盆是陆缘扩张引起的主动盆地的观点。     

Synchronous detachment folds and coeval sedimentation in the Prepyrenean External Sierras (Spain): a case study for a tectonic origin of sequences and systems tracts

This paper reports on the structural and sedimentary evolution of the middle to late Eocene of the Prepyrenean External Sierras (southern Pyrenees, Spain). The initiation, duration and kinematics of a set of growth structures that developed in a shallow marine depositional setting is documented. The detailed analysis of the syntectonic marine sediments not only confirms the already known east to west progression of deformation, but also reveals the continued growth of the early formed structures as later ones propagate towards the foreland. The sedimentary units coevally deposited with these growth structures are arranged in four depositional sequences. Their boundaries correspond to flooding surfaces which grade basinwards into correlative conformities. They are also indicated by the presence of both angular unconformities and onlap geometries. Each depositional sequence generally consists of two systems tracts. The lower one, or transgressive systems tract, is formed by up to 400 m of azoic marls deposited in outer ramp areas. The upper one, or highstand systems tract, mainly consists of shallow siliciclastic and carbonate facies, up to 200 m thick, deposited in middle to inner ramp areas. These depositional sequences are interpreted to be controlled by regional tectonic pulses. An increase of tectonic activity resulted in the flooding of the basin and in the subsequent deposition of a thick succession of nearly azoic blue marls (i.e. transgressive systems tract). The overlying highstand systems tract developed following periods of diminished tectonism, with the consequent growth and progradation of shallow carbonate platform facies.     

Episodic Paleozoic Adakitic Magmatism in the Eastern Tianshan, Southern Altaids: Implications for Petrogenesis and Geodynamics

The adakites rocks are typically considered as direct diagnostic evidence for subduction and continental growth, as well as the important evidence to determine the location of the major oceanic basin and evolution history of lost oceanic basins, which are longstanding conundrums for understanding the tectonics of the Paleo-Asian Ocean (PAO). As the processor of the Altaids, the PAO was a long-lived ocean, and its multiple subductions led to substantial continental growth in the Central Asian continent. Here we report our newly discovered Paleozoic adakitic rocks (granite and dacite) developed in Jijitaizi in the Eastern Tianshan of the southern Altaids to address the above issues. The Jijitaizi granite and dacite contain high concentrations of Sr (203–343 ppm) and low concentrations of Y (3.00–6.36 ppm) and Yb (0.23–0.62 ppm), indicative of the adakitic affinity. Zircon U-Pb dating results show that the adakitic rocks were formed in three episodes at 372.26 ± 1.51 Ma, 356.02 ± 1.11 Ma and 334.51 ± 0.68 Ma from the Late Devonian to Carboniferous. The rocks are depleted in Nb, P and Ti, and possess variable Mg# values (28.51–57.22), high Na2O/K2O ratios (1.39–4.94), as well as relatively depleted Hf isotopic compositions, all of which point to a subduction origin from slab melting. Our new finding with the date from the Jijitaizi complex (ca. 321 Ma) suggests continuing subduction of the Paleo-Asian oceanic slab beneath the Central Tianshan from the Late Devonian to Carboniferous. Our work provides a solid line of key evidence demonstrating that continuous oceanic subduction of the major PAO evolved into its mature stage following subducting beneath the Central Tianshan from ca. 372 to 321 Ma.     

Early Paleozoic gabbro-granitoid associations in eastern segment of the Mongolian-Okhotsk foldbelt (Amur River basin): Age and tectonic position

Magmatic rocks of the Pikan and Un’ya massifs situated in eastern segment of the Mongolian-Okhotsk foldbelt are studied using isotopic-geochronological (U-Pb zircon dating) and geochemical methods. Two rock complexes different in age are recognized in the Pikan massif: the high-Al gabbro-tonalite association of the Middle Ordovician (468 ± Ma) and granodiorite-granite association of the Late Silurian-Early Devonian (415 ± 7 Ma). The Late Ordovician age (454 ± 5 Ma) is established for leucocratic granites of the Un’ya massif. As is suggested, the Pikan and Un’ya massifs are “allogenic blocks” detached from continental framework of the Mongolian-Okhotsk foldbelt and tectonically emplaced into the foldbelt structure at the last stage of its development.     

Structure,magmatism, and Paleozoic tectonic evolution of the Uakit Zone in the context of the formation of the Angara-Vitim batholith in the western Transbaikal region

The problem of the tectonic evolution of the Angara-Vitim batholith, the largest igneous complex of the western Transbaikal region, is discussed. This problem is still far from an unequivocal solution; however, it is evident that the Late Paleozoic stage was crucial in the evolution of the Angara-Vitim batholith, and precisely this stage has determined the main structural features of the western Transbaikal region. The geodynamics that controlled the batholith formation in the Late Paleozoic is exemplified in the Uakit Zone. New data are presented on the stratigraphy of country rocks and on the age and composition of the Vitimkan granitoid complex, the most abundant complex in the batholith. It has been shown that the main tectonic events in the Uakit Zone occurred from the Devonian to the Late Carboniferous. The ensialic orogeny in the form of a reduced Wilson cycle that developed under within-plate conditions was the main mechanism of structure formation. The effect of collision is a self-sufficient mechanism of superplume evolution provided by the origin and passing away of particular, relatively short lived streams of the superplume. The short-lived rifts (aulacogens) arose above the ascending streams during the most active development of superplume and then closed up under tangential compression caused by spatially conjugated younger plumages. The closure of rifts was controlled by pseudosubduction and particular collision. As a result, the mosaic block megastructure was created, being underlain by the hydrated mantle necessary for extensive granite formation. Such a mechanism probably was dominant during a “vague time” in the Riphean geological history of the Earth after the breakdown of Rodinia.     

新疆北部晚古生代大规模岩浆作用与成矿耦合关系研究主要进展及成果

新疆北部晚古生代地质构造演化复杂,岩浆作用形式多样,造就了大规模的成矿作用.本研究紧紧围绕岩浆铜镍矿床、斑岩型铜(钼)矿床及火山岩型磁铁矿矿床,从含矿岩体的岩浆起源、岩浆演化及成矿特点,系统研究深部相应岩浆活动的地质过程.通过典型矿床的深入剖析,建立相应矿床类型的成矿模式,破解制约找矿突破的控制因素,系统阐述了板块构造与地幔柱体制叠加并存的地质特征与成矿表现.鉴于塔里木地幔柱的活动特点和成矿表现,将其与新疆北部三类主要矿床类型建立关联,对比岩石学、年代学及地球化学特点,发现其成矿类型与塔里木地幔柱及板块构造存有密切关系,可能是两种构造体制叠加并存的结果.塔里木克拉通深部熔融的地幔物质,围绕刚性塔里木克拉通边缘不断上涌,并与表壳物质发生交换,随着俯冲板块的持续和减弱,深部上涌的地幔物质不断加强,先后形成因深部地幔物质多寡而金属聚集的不同矿床类型.     

The Monte Orfano Conglomerate revisited: stratigraphic constraints on Cenozoic tectonic uplift of the Southern Alps (Lombardy, northern Italy)

The Monte Orfano Conglomerate (MOC), exposed in the foothills of the Southern Alps (northern Italy), is one of the few outcrops of sediments documenting the Cenozoic tectonic evolution of the Alpine retrowedge. Calcareous nannofossil biostratigraphy allowed us to constrain the upper part of the MOC, formerly attributed to the Early-Middle Miocene in the type-locality, to the earliest Miocene (Neogene part of the NN1 nannofossil zone). A likely latest Oligocene age is therefore suggested for the bulk of the underlying conglomerates, whose base is not exposed. Deposition of the MOC can be placed within the post-collisional tectonic uplift of the Alps, documented in the Lake Como area by the Como Conglomerate (CC) at the base of the Gonfolite Lombarda Group, and supports the correlation with Upper Oligocene clastic sediments cropping out further to the East, in the Lake Garda and in the Veneto-Friuli areas (“molassa”). The remarkable difference in petrographic composition between the western (CC) and eastern (MOC) clastics deposited in the Alpine retro-foreland basin highlights the synchronous tectonic activity of two structural domains involving different crustal levels. Whilst the bulk of the CC, that straddles the Oligocene/Miocene boundary, records largely the tectonic exhumation of the Alpine axial chain crystalline complexes, the coeval MOC consists of detritus derived from the superficial crustal section (Triassic to Paleogene sedimentary rocks) of the Alpine retrowedge and constrains the onset of the post-collisional deformation phase of the Southern Alps as not younger than the Late Oligocene.     

Quantitative subsidence-uplift analysis of the Bajo Segura Basin (eastern Betic Cordillera, Spain): tectonic control on the stratigraphic architecture

The Bajo Segura Basin is located in the eastern Betic Cordillera, at present connected with the Mediterranean Sea to the east. It has a complete stratigraphic record from the Tortonian to the Quaternary, which has been separated into six units bounded by unconformities. This paper is concerned with the northern edge of the basin, controlled by a major strike–slip fault (the Crevillente Fault Zone, CFZ), where the most complete stratigraphic successions are found. The results obtained (summarised below) are based on an integrated analysis of the sedimentary evolution and the subsidence-uplift movements. Unit I (Early Tortonian) is transgressive on the basin basement and is represented by ramp-type platform facies, organised in a shallowing-upward sequence related to tectonic uplift during the first stages of movement along the CFZ. Unit II (lower Late Tortonian) consists of shallow platform facies at bottom and pelagic basin facies at top, forming a deepening-upward sequence associated with tectonic subsidence due to sinistral motion along the CFZ. Unit III (middle Late Tortonian) is made up of exotic turbiditic facies related to a stage of uplift and erosion of the southern edge of the basin. Unit IV (upper Late Tortonian) consists of pelagic basin facies at bottom and shallow platform facies at top, defining a shallowing-upward sequence related to tectonic uplift during continued sinistral movement on the basin-bounding fault. Units V (latest Tortonian–Messinian) and VI (Pliocene–Pleistocene p.p.) consist of shallowing-upward sequences deposited during folding and uplift of the northern margin of the basin. No definitive evidence of any major eustatic sea-level fall, associated with the ‘Messinian salinity crisis’, has been recorded in the stratigraphic sections studied.     

Late Ordovician (Sandbian) brachiopods from the Mweelrea Formation,South Mayo,western Ireland: stratigraphic and tectonic implications

A Late Ordovician (Sandbian), Scoto‐Appalachian brachiopod fauna from the Mweelrea Formation in western Ireland confirms a location for the South Mayo Trough adjacent to the Laurentian margin, characterized at this time by a succession of marine excursions over fluviatile environments. The new, younger biostratigraphical data help to constrain the timing of late Grampian folding of this part of the South Mayo Trough. Copyright © 2010 John Wiley & Sons, Ltd.     

Non‐uniqueness and symmetry in stratigraphic interpretations: A quantitative approach for determining stratal controls

Different combinations of stratal controls could produce identical sequence architectures. Consequently, interpretations of the stratigraphic record, for example to infer palaeo‐climate and eustatic sea‐level history, suffer from non‐uniqueness. However, variations in the multiple controls can be encapsulated through discovery of all possible solutions to an interpretation. As this paper demonstrates, a single solution can be directly transformed into an alternative solution that leaves the expected geological outcomes unaltered, which can be regarded as the existence of symmetry in the interpretation. Repetitive application of the symmetry method can therefore allow additional solutions to be rapidly derived given an existing solution. The proposed method has been adapted to a stratigraphic forward model for interpreting the Baltimore Canyon (USA) stratigraphy. Modelling results have indicated the ranges of changes in relative sea‐level, sediment supply and subaerial erosion from Oligocene to Mid‐Miocene. Using these limits, it is possible to determine what appears to be true in the palaeo‐history, even when a solution is not unique.     

西准噶尔成矿带晚古生代花岗岩类岩浆活动及其构造意义

中亚造山带是晚古生代地壳显著生长与大规模成矿的重要地区。本文采集了中亚造山带西部的西准噶尔成矿带哈图-别鲁阿嘎希及其附近地区11个岩体共33件花岗岩类样品,对其开展了岩石地球化学与同位素示踪等研究,厘定了该地区晚古生代岩浆活动的特点与大地构造环境,并与哈萨克斯坦境内的巴尔喀什成矿带晚古生代岩浆活动进行了对比。研究表明,哈图地区晚石炭世花岗岩类主要为后碰撞伸展构造环境的A型花岗岩类,别鲁阿嘎希等地区存在洋内俯冲与岛弧环境的埃达克岩,显示了西准噶尔晚古生代构造环境时空变化的复杂性。该地区花岗岩类εNd(t)值较高(+4.62~+7.53)、εSr(t)值为(-57.61~+18.21),具有中亚造山带花岗岩类的共同特征,为古生代增生的新生陆壳,其源区与亏损地幔组分具有亲缘关系,这与巴尔喀什成矿带东段的花岗岩类具有一致性。花岗岩的~(206)Pb/~(204)Pb、~(207)Pb/~(204)Pb和~(208)Pb/~(204)Pb比值范围分别为18.2776~19.1677、15.5260~15.5796和38.2080~39.0821,为造山带花岗岩类。     

甘孜-理塘蛇绿混杂岩带晚三叠世构造演化——来自理塘地区勇杰岩体地球化学、年代学的制约

甘孜-理塘蛇绿混杂岩带是西南三江构造带的重要组成部分,经历了古特提斯构造体系的演化,形成完整的沟-弧-盆体系。义敦岛弧属甘孜-理塘弧盆系范畴,位于甘孜-理塘缝合带西侧。对义敦岛弧勇杰岩体开展详细的岩石学、地球化学、年代学研究,为甘孜-理塘洋盆晚三叠世构造演化研究提供新的证据。义敦勇杰岩体黑云母二长花岗岩和花岗质细晶岩的锆石UPb年龄分别为214.2±1.4Ma和206.2±1.8Ma,整体属高钾钙碱性弱过铝质花岗岩;稀土元素配分曲线具轻稀土元素相对富集、重稀土元素相对平坦的特征,负Eu异常明显;微量元素表现为大离子亲石元素Rb、Ba、Th、U、K相对富集,高场强元素Nb、Ta、P、Ti亏损的特征,显示勇杰岩体黑云母二长花岗岩明显具俯冲型花岗岩的特征,而花岗质细晶岩具有碰撞型花岗岩的特征。总体反映晚三叠世甘孜-理塘洋盆从俯冲至碰撞的地球动力学背景,应属理塘蛇绿混杂岩带碰撞造山过程的产物。     

Provenance and tectonic settings of the Late Paleozoic sandstones in central Inner Mongolia,NE China: Constraints on the evolution of the southeastern Central Asian Orogenic Belt

Late Paleozoic sedimentary strata outcrop extensively in central Inner Mongolia, and are a key to understanding the tectonic evolution of the southeastern Central Orogenic Belt. A combined analysis of petrography, whole-rock major and trace element, and Nd isotope is carried out on representative sandstones from the Late Paleozoic sedimentary strata (420–270 Ma). The sandstones are mainly wackes and litharenites in lithology, with low SiO₂/Al₂O₃ of 2.85–9.47 (averagely 5.22) and poor textural and compositional maturities, implying short sediment transportation between the depositional basins and provenances. The trace element compositions are generally comparable to that of the average upper continent crust (UCC), with negatively-sloping chondrite-normalized rare earth element distribution patterns ((La/Yb)_N = 3.43–11; averagely 6.94) and flat UCC-normalized trace element distribution patterns. The Nd isotopic compositions show great variation (Ԑ_Nd(t) = −5.01 to 5.35) with depositional time of the sandstones, and coincide well with the arc magmatic phases in central Inner Mongolia. The geochemical signatures of the sandstones indicate that the dominant provenances are intermediate to felsic arc magmatic rocks that have ages approximating the deposition, although old, recycled sediments may have made a minor contribution. An active continental arc setting during the Late Paleozoic in central Inner Mongolia, controlled by the northward subduction of the Paleo-Asian oceanic slab, was the most likely depositional tectonic setting of the sandstones. This active continental arc setting continued to at least 270 Ma, implying that the final closure of the Paleo-Asian Ocean along the Solonker suture zone most likely occurred sometime during the Late Permian to Early Triassic. The northward subduction of the Paleo-Asian Ocean is likely of West Pacific-style, in which the present-day Baolidao arc has a close genetic link with the South Mongolian microcontinent and, likely, the former originally formed as the arc margin of the latter.